1. Technical Field of the Invention
The present invention relates to a driving apparatus for use in four-wheel drive vehicles for transmitting power from a drive source to steerable wheels and nonsteerable wheels via a main HST (hydraustatic transmission).
2. Prior Art
When four-wheel drive vehicles having steerable wheels and nonsteerable wheels respectively at a front position and a rear position of the vehicle are turned, a difference occurs in turning radius between the steerable wheel and the nonsteerable wheel. For example, when the vehicle shown in FIG. 1 and having rear wheels serving for steering is turned, the rear wheel turns along a path of a greater radius than the front wheel. If the front and rear wheels are rotated at the same speed, therefore, the rear wheel skids on the terrain. Such a skid of the rear wheel roughs the terrain or causes wear on the rear wheel. Especially with vehicles adapted to travel on lawns like lawn mowers, roughing the terrain is undesirable.
Already proposed to preclude this drawback are (1) a driving apparatus having a one-way clutch provided in the path of power transmission to the steerable wheels for transmitting a driving power to the steerable wheel and also permitting the steerable wheel to rotate at a higher speed than the nonsteerable wheel when the vehicle is turned, and (2) a driving apparatus wherein the path of power transmission to the steerable wheels is provided with a gear transmission unit for giving an increased speed. The gear transmission unit is brought out of operation when the vehicle is advanced straight, or is operated to rotate the steerable wheel at a higher speed than the nonsteerable wheel when the vehicle is turned.
However, the driving apparatus (1) has the drawback the vehicle fails to fully exhibit the performance of four-wheel drive since the nonsteerable wheels only are driven as in a two-wheel drive vehicle when the vehicle is turned. This entails another drawback that the vehicle is impaired in running performance when turning with a small radius since the nonsteerable wheels only are driven for turning.
Further with the driving apparatus (2), the steerable wheel is rotatable only at two speeds, i.e., a high speed and the same speed as the nonsteerable wheel. On the other hand, the difference between the steerable wheel and the nonsteerable wheel in turning radius is proportional to the steering angle of the steering wheel to be handled by a driver. Thus, in order to effectively prevent the steerable wheels from skidding during turning of the vehicle, there is a need to vary the rotational speed difference between the steerable wheel and the nonsteerable wheel. Accordingly the driving apparatus (2) which affords only two different rotational speeds to the steerable wheels fails to fully preclude the steerable wheels from skidding. Further although the driving apparatus is capable of providing two different speeds, i.e., a high speed and the same speed, for the steerable wheel, it is impossible to increase the rotational speed difference between the steerable wheel and the nonsteerable wheel because the rotational speed of the nonsteerable wheel is constant. If the vehicle is turned with a small radius, i.e., if the difference between the steerable wheel and the non and the nonsteerable wheel in turning radius is great, it is difficult to obtain such a speed difference as to offset the radius difference.
Also known is a driving apparatus similar to the apparatus 1 and having a transmission unit for giving an increased speed which unit comprises a pair of pulleys and a belt reeved around the pulleys, the pulleys being variable in effective diameter according to the rotation angle of the steering wheel at the driver""s seat. However, although adapted to vary the rotational speed of the steerable wheel, the apparatus also encounters difficulty in increasing the rotational speed difference between the steerable wheel and the nonsteerable wheel since the rotational speed of the nonsteerable wheel is constant.
Further, in a vehicle with a four-wheels steering system, the resembling problem occurs. That is, the turning radius of front ground wheel and rear ground wheel is usually different to each other. Therefore, the adjustment between the rotational speed of the front wheels and the rear wheels is necessary.
An object of the present invention, which has been accomplished to overcome the foregoing problems, is to provide a driving apparatus for use in four-wheel drive vehicles, the apparatus being adapted to effectively prevent the steerable wheels from skidding while transmitting power to the steerable wheels when turning the vehicle.
To fulfill the foregoing object, the present invention provides a driving apparatus for a vehicle for transmitting power from a drive source installed in a body of the vehicle to steerable wheels and nonsteerable wheels via a main HST, the driving apparatus comprises: a transmission unit for receiving a rotational output from the main HST, a steerable wheel drive shaft and a nonsteerable wheel drive shaft for receiving a rotational output from the transmission unit and transmitting the rotational output respectively to an axle for driving the steerable wheels and an axle for driving the nonsteerable wheels, and a differential unit for rotating the steerable wheel drive shaft at an increased speed and rotating the nonsteerable wheel drive shaft at a decreased speed according to the steering angle of the steering wheel.
Preferably, the driving apparatus can be so constructed that the steerable wheel drive shaft and the nonsteerable wheel drive shaft are arranged on approximately the same axis and spaced apart from each other at opposed ends thereof, the transmission unit comprising: a main drive shaft disposed between the opposed ends of the steerable wheel drive shaft and the nonsteerable wheel drive shaft on the same axis as the two shafts, a driving power transmission mechanism for transmitting the rotational output of the main HST to the main drive shaft, and a steerable wheel planetary gear unit and a nonsteerable wheel planetary gear unit for transmitting the rotation of the main drive shaft respectively to the steerable wheel drive shaft and the nonsteerable wheel drive shaft so as to rotate the wheel drive shafts in the same direction, the steerable wheel planetary gear unit having a first sun gear mounted on the main drive shaft, a first outer wheel surrounding the first sun gear, a first inner gear provided on an inner periphery of the first outer wheel, first planetary gears arranged between the first sun gear and the first inner gear, and a first carrier supported on the steerable wheel drive shaft nonrotatably relative thereto and rotatable with the revolution of the planetary gears, the nonsteerable wheel planetary gear unit having a second sun gear mounted on the main drive shaft, a second outer wheel surrounding the second sun gear, a second inner gear provided on an inner periphery of the second outer wheel, second planetary gears arranged between the second sun gear and the second inner gear, and a second carrier supported on the nonsteerable wheel drive shaft nonrotatably relative thereto and rotatable with the revolution of the second planetary gears.
The driving apparatus can be so constructed that the differential unit comprises: a differential HST for receiving the power from the drive source and outputting a differential rotational drive force, and a second differential power transmission mechanism for receiving the rotational output from the differential HST and giving the first outer wheel and the second outer wheel respective additional rotations in opposite directions to each other, the differential HST being adapted not to output the rotational drive force when the vehicle is advanced straight and to output the rotational drive force with a number of revolutions in accordance with the steering angle of the steering wheel when the vehicle is turned, the differential power transmission mechanism being adapted to give the first outer wheel an additional rotation of the same direction as the rotation of the first sun gear rotated by the main HST and to give the second outer wheel an additional rotation opposite in direction to the rotation of the second sun gear rotated by the main HST.
Further the driving apparatus can be so constructed that the differential HST has a hydraulic pump and a hydraulic motor, at least one of the hydraulic pump and the hydraulic motor being of the variable displacement type which is variable in displacement by operating a displacement altering member, the differential unit comprising a link mechanism coupling the steering wheel to the displacement altering member, the link mechanism being adapted to reduce approximately to zero the discharge rate of one of the hydraulic pump and the hydraulic motor which has the displacement altering member when the steering wheel is in a posture to advance the vehicle straight and to increase the discharge rate as the steering angle of the steering wheel increases when the steering wheel is in a posture to turn the vehicle.
Alternatively, the driving apparatus can be so constructed that the steerable wheel drive shaft and the nonsteerable wheel drive shaft are arranged on approximately the same axis and spaced apart from each other at opposed ends thereof, the transmission unit comprising a first differential gear mechanism having a pair of first sun gears fixedly mounted on the steerable wheel drive shaft and the nonsteerable wheel drive shaft, respectively, at their opposed ends, the first differential gear mechanism comprising a first casing covering the pair of first sun gears and rotatably supported on the drive shafts, a first ring gear provided externally on the first casing for receiving the rotational output from the main HST, and a plurality of first planetary gears meshing with the pair of first sun gears and rotatably supported by a shaft fixedly provided inside the first casing and extending diametrically of the casing, the differential unit comprising a differential HST for receiving the power from the drive source and outputting a differential rotational drive force, a second differential gear mechanism for receiving the rotational output from the differential HST, and an adjusting transmission mechanism for transmitting an output from the second differential gear mechanism to the steerable wheel drive shaft and the nonsteerable wheel drive shaft, the second differential gear mechanism comprising a pair of rotary shafts supported by a housing of the driving apparatus, arranged on approximately the same axis and spaced apart from each other at opposed ends thereof, a pair of second sun gears fixedly mounted on the respective rotary shafts at their opposed ends, a second casing covering the pair of second sun gears and rotatably supported on the pair of rotary shafts, a second ring gear provided externally on the second casing for receiving the rotational output from the differential HST, and a plurality of second planetary gears meshing with the pair of second sun gears and rotatably supported by a shaft fixedly provided inside the second casing and extending diametrically of the second casing, the adjusting transmission mechanism comprising two gear trains provided for the steerable wheel drive shaft and the nonsteerable wheel drive shaft respectively and each comprising a drive gear fixed to the rotary shaft, and a driven gear fixed to the corresponding drive shaft, at least one of the two gear trains having at least one intermediate gear so as to render the two driven gears rotatable in directions different from each other, the drive gear and the driven gear of each of the gear trains being equal in diameter.
Preferably, each of the driving power transmission mechanism and the differential power transmission mechanism can be a power transmission gear mechanism.
As explained above, the driving apparatus embodying the invention for use in vehicles comprises a transmission unit for receiving a rotational output from a main HST, a steerable wheel drive shaft and a nonsteerable wheel drive shaft for receiving a rotational output from the transmission unit, and a differential unit for rotating the steerable wheel drive shaft at an increased speed and rotating the nonsteerable wheel drive shaft at a decreased speed according to the steering angle of the steering wheel of the vehicle, so that when the vehicle is turned, the steerable wheels can be effectively precluded from skidding with the vehicle propelled by four-wheel drive.
The transmission unit comprises a main drive shaft for receiving the rotational output of the main HST via a driving power transmission mechanism, and a steerable wheel planetary gear unit and a nonsteerable wheel planetary gear unit for transmitting the rotation of the main drive shaft respectively to the steerable wheel drive shaft and the nonsteerable wheel drive shaft; and the differential unit comprises a differential HST for outputting a rotational drive force in accordance with the steering angle of the steering wheel, and a driving power transmission mechanism for receiving the rotational output of the differential HST, giving the steerable wheel drive shaft an additional rotation for a speed increase and giving the nonsteerable wheel drive shaft an additional rotation for a speed reduction. When the driving apparatus is thus constructed, the rotational speed of the steerable wheels and the nonsteerable wheels can be controlled with good stability for turning the vehicle.
Further when the differential HST is of the variable displacement type having a movable swash plate or like displacement altering member, and the steering wheel is operatively connected to the displacement altering member by a link mechanism, the steering angle of the steering wheel can be operatively related to the rotational output of the differential HST with good stability.
When the driving power transmission mechanism and the differential power transmission mechanism are each a power transmission gear mechanism, an improved transmission efficiency can be achieved.
Another aspect of the present invention provides a driving apparatus for a vehicle for transmitting power via a main HST from a drive source installed in a body of the vehicle to a first pair of driving wheels and a second pair of driving wheels, the pairs of wheels being positioned at the front and rear of the vehicle body and at least one pair of the pairs of driving wheels being steerable, the driving apparatus being characterized in that the driving apparatus comprises: a transmission unit for receiving a rotational output from the main HST, a first drive shaft and a second drive shaft for receiving a rotational output from the transmission unit and transmitting the rotational output respectively to an axle for driving the first pair of wheels and an axle for driving the second pair of wheels, and a differential unit for rotating the first drive shaft and the second drive shaft at a speed adjusted according to the steering angle of a steering wheel to be handled by a driver.
Therefore, for a vehicle with a four-wheels steering system, i.e. for a vehicle with a first pair of driving steerable wheels and a second pair of driving steerable wheels positioned at the front and rear of the vehicle body, the differential unit can be so structured that the differential unit rotates the first drive shaft and the second drive shaft at a speed adjusted according to the steering angle of a steering wheel to be handled by a driver. According to so structured driving apparatus, the pair of the front wheels and the pair of the rear wheels are driven by the first and second drive shafts rotating with adjusted speed according to the steering angle of the steering wheel, and thus precluded from skidding even when the difference between the turning radius of front wheel and rear wheel occurs with turning of a vehicle.
The invention will be further clarified by the description of embodiments with reference to the following accompanying drawings. The invention is not limited to these embodiments, but various modifications are possible without deviation from the scope of the claims.